1. Field of the Invention
The present invention relates in general to materials used during integrated circuit fabrication, and more particularly to materials used as diffusion barriers.
2. Background
A gate electrode is a structure commonly found in an integrated circuit. One fabrication technique of a gate electrode begins with the formation a gate oxide film on a semiconductor substrate. Following this step, a polysilicon layer is formed on the gate oxide film. A tungsten silicide layer is then deposited over the polysilicon layer using a chemical vapor deposition (CVD) process. The CVD process may include the use of a fluorine-containing gas, such as tungsten hexafluoride (WF6). Therefore, during CVD, fluorine atoms can be incorporated into the tungsten silicide layer.
Once the gate oxide film, the polysilicon layer, and the tungsten silicide layer have been formed on the substrate, the device is annealed. During this anneal, fluorine atoms undesirably diffuse from the tungsten silicide layer through the polysilicon layer to the gate oxide film.
To reduce the diffusion of fluorine atoms during the anneal, attempts have been made to incorporate a diffusion barrier layer under the tungsten silicide layer during the fabrication of gate electrodes. Conductive diffusion barrier layers comprise materials such as titanium nitride, titanium tungsten, or tantalum nitride. Although these materials inhibit fluorine diffusion to some extent, they still allow a substantial amount of fluorine to diffuse from the metal layer to the gate oxide film.
When fluorine atoms diffuse to the gate oxide film, they react with the gate oxide film in a manner that increases its electrical thickness. Furthermore, fluorine diffusion can lower breakdown voltage and increase defect density. Fluorine diffusion can also cause device degradation, such as a shift in threshold voltage or a decrease in saturation current.